With more than 42 million HIV-infected individuals worldwide, HIV infection continues to be a significant health problem. Notably, women comprise nearly half of the infected population, with a staggering and increasing rate of HIV infection in many regions of the world. This has spurred researchers to develop preventative measures to slow or stop the male to female sexual transmission of HIV with agents called microbicides. Moreover, the sexual transmission of HIV is often aided by the presence of other factors. Data from more than 30 epidemiological studies indicate that herpes simplex virus type-2 (HSV-2) infection is associated with about a two- to four-fold increase in the risk of HIV-1 acquisition. Due to the prevalence of HSV-2 in the general population, we recognize this as a significant aggravating factor for the transmission of HIV. The overall goal for the project is to develop and optimize a multifunctional combination microbicide that is safe and effective in preventing HIV transmission even in populations with high HSV-2 prevalence. We will evaluate the safety and efficacy of this candidate microbicide in a SHIV rhesus macaque model that mimics HIV transmission in HSV infected humans. In year 1 and 2, there are two complementary yet independent goals that we will work toward simultaneously. The first goal is to design and formulate a bioresponsive combination microbicide composed of a self- assembling gel that is a semisolid at vaginal pH and liquefies at seminal pH. The gels will be loaded with the antiviral nucleoside inhibitors Tenofovir (TNFV) and Acyclovir (ACV), and a viral neutralizing agent CAP (cellulose acetate phthalate). The gels will be tested for release rate, antiviral activity and toxicity in vitro and in vivo. From this data, two lead formulations will be identified. The second goal is to develop and validate a simian HIV (SHIV infection in a primate model that mimics HIV infection in humans). To address the effectiveness of an anti-HIV microbicide in a population with high incidence of HSV infection, we will test the combination microbicide in a viral co-infection primate model. We will first establish and validate a SHIV and HSV dose in macaques that produces virus shedding and disease development. In R33 phase, this primate model will be used to evaluate efficacy and safety of the most promising microbicide formulation containing virus neutralization agent (CAP) and antiviral (TNFV and ACV) in providing maximum protection against HIV transmission. Such experiments, planned for years 3-5 will determine dose response, dose frequency and dose duration that provide maximum protection against HIV transmission.